1. Field of the Invention
The present invention relates to a synchronization apparatus and method for an OFDM-FDMA/CDMA/TDMA system, and more particularly, to a synchronization apparatus and method for improving timing estimation performance in an OFDM-FDMA/CDMA/TDMA system, which can correctly estimate symbol timing through a more correct timing metric using a guard interval of a preamble.
2. Description of the Related Art
Research on the 4th Generation (4G) communication system (which is the next-generation communication system) has been conducted to provide users with services having various Qualities-of-Service (QoSs) at a data rate of about 100 Mbps.
The current 3rd Generation (3G) communication system generally supports a data rate of about 384 Kbps in an outdoor channel environment with a relatively poor channel environment, and supports a data rate of up to 2 Mbps even in an indoor channel environment with a relatively good channel environment.
Meanwhile, a wireless local area network (WLAN) system and a wireless metropolitan area network (WMAN) system generally support a data rate of 20-50 Mbps.
Accordingly, research is being conducted on a new communication system that can support high-speed services, which the 4G communication system is intended to provide, by guaranteeing the mobility and QoS in the WLAN system and the WMAN system having a relatively high data rate.
As an approach to achieving these purposes, Orthogonal Frequency Division Multiplexing (OFDM) scheme and Orthogonal Frequency Division Multiple Access (OFDMA) scheme are taken into consideration.
The OFDM is a kind of multicarrier modulation scheme and exhibits excellent performance in multi-path and mobile environments. Because of these advantages, the OFDM is regarded as a modulation scheme suitable for terrestrial digital TV and digital sound broadcasting.
An existing Institute of Electrical and Electronics Engineers (IEEE) 802.11 WLAN uses Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) to support a data rate of 2 Mbps at 2.4 GHz ISM (Industrial, Scientific, and Medical) bands.
This standard, however, could not meet the requirement of a highly increasing data rate. Therefore, IEEE 802.11a and IEEE 802.11b was approved as a new physical layer standard in 1999.
IEEE 802.11b supports a date rate of 11 Mbps at 2.4 GHz bands by using a complementary Code Keying (CCK) scheme, an extended version of DSSS. IEEE 802.11b is commercialized and is wide spread.
Meanwhile, IEEE 802.11a adopted OFDM modulation scheme at Unlicensed National Information Infrastructure (U-NII) band of 5 GHz in order to overcome the limitation of the DSSS scheme and obtain a higher data rate.
A convolutional encoder having a coding rate of 1/2, 2/3 and 3/4 is used for offset correction, and a 1/2 Viterbi decoder is used for decoding a convolutional code. Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (16-QAM), and 64-QAM are used for subcarrier modulation.
Such an OFDM system must consider frequency offset and symbol synchronization in order to correctly demodulate received signals. If a correct starting point of symbol is not found, inter-symbol interference (ISI) occurs. Consequently, the received signals cannot be correctly recovered.
Generally, the starting point of the symbol is found using correlation values between received signal sequences. The correlation values are obtained using a specific preamble sequence.
The preamble is a single used for synchronizing a transfer timing between two or more systems in a network communication. A proper timing makes it possible for all systems to correctly analyze the start of data transfer.
The preamble includes a short preamble for coarse frequency synchronization and a long preamble for fine frequency synchronization, which are connected each other.
FIG. 1 illustrates a preamble structure of an 802.16a/d/e system and WiBro system.
A downlink preamble of an OFDMA-based portable Internet system is used for initial time synchronization, frequency offset estimation, and cell search. After inverse Fourier transform (IFFT) processing, the downlink preamble has harmonics repeated three times in time domain.
A specific pseudorandom noise (PN) code is BPSK-modulated and then transmitted and subcarriers constructing the preamble are repeated by Np (Np=NFFT/3 where NFFT is an FFT size) in time domain.
FIG. 2 is a graph illustrating a timing metric of a conventional symbol timing estimation algorithm. The timing metric has a flat period as much as a guard interval.
A frame starting point and an initial symbol timing are acquired at a position where the timing metric has a peak value. In the ideal case in which no influence of channel or interference signal exists, the frame starting point and the initial symbol timing are acquired within the guard interval. However, there is a great probability that a wrong frame timing is acquired out of the guard interval due to the uncertainty at a boundary portion of the timing metric.
FIG. 3 illustrates a constellation diagram of a traffic channel using the conventional symbol timing estimation algorithm.
Even though the symbol timing is acquired within the guard interval by using a conventional symbol timing estimation algorithm, the accuracy of the estimated timing is reduced.
The timing estimation error appears in a form of phase rotation. Because the timing estimation error is compensated through a channel estimator, it is negligible. However, when the timing estimation error is large, the channel estimator cannot correct the entire error. Thus, the result is given like the constellation diagram of FIG. 3 and the signal-to-noise ratio (SNR) of the received signal is degraded.